Surgical removal of epileptogenic brain is indicated for treatment of many medically refractory focal seizure disorders. One of the important factors in providing good results from such surgery is the degree of accuracy in identifying epileptogenic foci. Various methods have been used in attempting to determine epileptogenic foci, and all, of course, involve sensing of cortical electrical activity using electrical contacts of various kinds applied in various ways.
In recent years many epilepsy centers have used intracranial recording techniques to better define regions of cortical epileptogenicity. Intracranial sensing techniques have used, broadly speaking, two different kinds of electrode members for engagement with brain tissue. Such different kinds of brain-contact devices include depth electrodes and flexible flat members which are known either as strip or grid electrodes, depending primarily on whether they have one or more rows of contacts.
These kinds of intracranial brain-contact devices each have a dielectric (non-conductive) base member on which an array of electrical contacts are mounted in spaced fashion. Separate leads extend from each of the contacts through the dielectric base member and therefrom to connectors and monitoring equipment which form no part of this invention.
Depth electrodes typically have contact rings sleeved over and spaced along a dielectric tubular member, with the leads extending inside the tube in a direction away from the distal end of the depth electrode. An example of depth electrodes is shown in U.S. Pat. No. 4,245,645 (Arseneault et al.).
Subdural strip and grid electrodes each have an array of contacts mounted on a sheet-like flat dielectric base member. Such contacts and the leads therefrom are usually held between two thin layers of dielectric material which are joined as one in the assembly process. An example of subdural electrodes is shown in U.S. Pat. No. 4,735,208 (Wyler et al.).
Depth electrodes penetrate deep into the brain tissue in direct contact with such tissue, while strip or grid electrodes are placed subdurally in direct contact with brain tissue at the surface of the brain, without penetrating brain tissue.
For each type of tissue-engagement member used in the prior art for monitoring electrical activity in the brain, the procedures for placement and hookup are of great importance. Accuracy of placement puts the contacts in the most advantageous positions for the period of observations which follows.
Observation periods often extend for days, one to three weeks being common. During such period the positions of the brain-contact devices can change to some extent. Furthermore, in some cases physicians may be somewhat uncertain for various reasons about precise locations of brain-contact devices even immediately after insertion.
Changes in electrode position are particularly likely for subdural strip and grid electrodes. Such subdural electrodes, unlike depth electrodes which are lodged in tissue, are somewhat free to move in their position between the dura and the brain tissue. Patient activities during an extended observation period can make such changes in position more likely.
Knowing the precise locations of the contacts of such electrical brain-contact devices is essential for accurate interpretation of the electrical readings which they sense. Electrical discharges picked up by intracranial contacts can be accurately associated with a specific location in the brain only to the extent that the precise locations of the contacts vis-a-vis the brain are known. Accurate knowledge about contact positions is essential for accurate determination of epileptogenic foci.
Since surgical removal of diseased brain cells is an intended subsequent course of action, accuracy in determining the location of diseased cells is of paramount importance. The substantial risks involved with removal of brain tissue are apparent. Accurate knowledge about the intracranial locations of the electrical contacts is critically important to successful subsequent surgical removal of diseased brain cells with minimum risk.
It is, therefore, desirable to make post-insertion checks on the precise location of the array of contacts of intracranial brain-contact devices, using x-rays. However, such location checks have been difficult at best primarily because of the nature of the electrical contacts. This is particularly the case with subdural strip and grid electrodes, in which the metal contacts, particularly the disks of subdural strip and grid electrodes, are themselves so thin and delicate that they cannot be seen or seen readily at desired x-ray powers.
There is a significant need for an improved electrical brain-contact device allowing a high degree of post-insertion confidence with respect to the intracranial positions of the electrical contacts.